The invention relates to a powered land vehicle including at least one spring-applied hydraulic release (SAHR) vehicle brake.
In such a brake first and second brake elements, mounted respectively on a rotatable part of the drive train of the vehicle such as a powered output shaft and a fixed part such as the vehicle frame are moveable between mutually engaged and separated configurations.
In the separated configuration the rotatable component is free to rotate, thereby permitting the transmission of drive from e.g. a diesel engine forming part of the vehicle to a ground-engaging member such as a wheel including a tyre. In the mutually engaged configuration the second element brakes rotation of the rotatable part supporting the first brake element with the result that the transmission of drive is resisted or prevented and therefore the vehicle as a whole is braked.
In almost all cases the second brake element is moveable towards and away from the first brake element. A resiliently deformable member such as a coil spring acts on the second brake element or on a member connected thereto in order to bias it into engagement with the first brake element. When the rotatable part of the drive train is an output shaft as aforesaid the first and second brake elements typically are the plates of a friction clutch.
The second brake element includes or is connected to a piston in a chamber having a connection to a hydraulic control circuit that is capable of applying pressure to the piston and hence indirectly to the second brake element itself. The hydraulic control circuit is selectively capable of being energised in order to assure separation of the first and second brake elements under normal circumstances. As a result the rotatable element is maintained able to rotate most of the time.
When the vehicle operator selects the brake function however control elements connected to the hydraulic control circuit cause venting of the fluid pressure acting on the piston. This in turn permits the resiliently deformable spring to urge the second brake element into engagement with the first, whereby application of the SAHR brake occurs in order to brake the vehicle regardless of the output of the engine or operative status of a variable-ratio transmission forming part of the drive train of the vehicle.
The SAHR vehicle brake serves primarily as a park brake that is intended reliably to brake the vehicle when it is stationary, whether the engine of the vehicle is running or not. The design of the SAHR lends itself to this use because the presence of the resiliently deformable spring maintains the vehicle in a braked condition even when the engine is switched off.
When as described the first brake element is an output shaft it is known to include additional control components that operate to disengage one or more drive clutches when the SAHR brake activates. This prevents the SAHR brake from having to work against the torque of the vehicle engine. In such a case the SAHR brake acts on a part of the output shaft that lies downstream, in the drive train, from any clutch permitting disengagement of drive.
SAHR vehicle brakes typically are fitted in large, powered land vehicles such as tractors, combine harvesters, forage harvesters and other harvesting machines, excavators, bulldozers, utility vehicles such as farm loaders and multipurpose farm vehicles, mobile cranes, and similar vehicles that while designed to perform specialised tasks usually in off-road situations nonetheless are intended to travel on roads between work sites. SAHR vehicle brakes are required in such vehicles to provide for operator and third party safety in both on-road and off-road situations. The invention is of utility in all vehicles of the general kind listed above, together with numerous further vehicle types.
As indicated the braking systems of large vehicles of the general category indicated typically are electrohydraulic or electropneumatic and therefore consist of braking elements the mutual engagement and separation of which are under the control of one or more hydraulic or pneumatic circuits. These circuits include circuit components such as valves the operating states of which are alterable between two or more configurations through the action of electrically powered elements such as solenoids that move moveable parts of the valves from one position to another.
Even though as mentioned it is known to provide for the disengagement of drive clutches in the drive train including the output shaft of such a vehicle, the duty of an SAHR brake that acts on an output shaft is extremely demanding. This is because even when freewheeling and not being driven under power the enormous masses of vehicles as listed above mean it requires considerable braking power to arrest them by braking their output shafts.
A disadvantage of braking a heavy vehicle in the manner described is that the heat generated in the SAHR brake can exceed the cooling capacity of any brake cooling system (that typically consists of a lubricant supplied to parts of the SAHR brake). When this happens damage may occur to the brake parts, shortening their service lives or even leading to failure of the SAHR brake.
As noted the SAHR brake normally is applied as a parking brake but under some (typically emergency) circumstances may be required to brake a moving vehicle. In such situations the vehicle may be moving at a considerable speed of up to e.g. 40 km/h (being the legal speed limit for tractors towing trailers in many EU countries) or even 80 km/h (being the actual maximum speed that some tractor and other agricultural vehicle designs can attain when not towing trailers).
In view of the foregoing there is a need in the art for improvements in SAHR brake performance, especially in cases in which the SAHR brake is arranged to act on an output shaft. The invention seeks to solve or at least ameliorate one or more drawbacks of prior art arrangements.